Yes you don't want to confuse those two!!!I don't know! My guess is that it is a very fine measurement to allow greater control over the weights of charges and bullets. 7000 grains to the pound.
Since you are in the Black powder section I will also note that in the BP world, Bullet weights are in grains but POWDER IS IN VOLUME!
This a is very important distinction. Black powder charges are measured in volumetric measure and not on a scale. Loading BP by weight could lead to unfortunate consequences.
Smokeless powder by weight
Black Powder by volume.